Add a volume (gain) property to QAudioInput.

Only implemented for PulseAudio so far, but the API does explain that
it's optional.

Change-Id: I4543a1c81d810fe92bb08f1ed13f3a3534a371e4
Reviewed-by: Ling Hu <ling.hu@nokia.com>

src/multimedia/audio/qaudiosystem.h

@@ -127,6 +127,8 @@ public:
     virtual QAudio::State state() const = 0;
     virtual void setFormat(const QAudioFormat& fmt) = 0;
     virtual QAudioFormat format() const = 0;
+    virtual void setVolume(qreal) {}
+    virtual qreal volume() const { return 1.0; }
 
 Q_SIGNALS:
     void errorChanged(QAudio::Error);